A frequency re-use factor is used in cellular systems to describe how many frequency channels are required for full coverage or, in other words, how close the same frequency can be re-used. In CDMA systems, where users are separated by code, re-use can be as low as 1, i.e. adjacent base stations can use the same frequency. In legacy TDMA systems, such as GSM, the re-use factor can be 7, for example. Modulation techniques being considered for fourth-generation wireless systems (known as “4G” systems) are multi-carrier systems, such as Orthogonal Frequency Division Multiplexing (OFDM) or MultiCarrier Code Division Multiple Access (MC-CDMA). Multi-carrier systems can be designed to have good tolerance against frequency-selective fading. Variable bandwidth can also be provided in such systems.
In 4G systems, capacities of 1 Gbps for a local area and 10 Mbps for wide area coverage have been envisioned. Such high data rates require a wide frequency band. However, even with very wide bandwidths on the order of 100 MHz, the spectral efficiencies needed for these capacities are extremely high. Targeting a spectral efficiency of 10 bits/sec/Hz, a band of 100 MHz is required for 1 Gbps throughput. Spectrum availability, though, is limited. In an optimal case, about 1 GHz of spectrum could be allocated for 4G services in some regions, but a typical allocation may be closer to 500 MHz.
To support multiple operators, the frequency re-use factor should be very low. Continuous coverage is difficult to provide in a system without spreading, when there is interference from neighbors in the cell borders. FIG. 1 illustrates a system simulation gain that can be achieved with the increase of frequency re-use from 1 to 4. The observed improvement of 9 dB is very significant and can make the difference between a system having or not having a continuous coverage. The gain increases further if the increased range of the narrowband carrier is taken into account.
U.S. Pat. No. 6,091,955, assigned to the same assignee as the present application, describes techniques for increasing capacity in GSM systems by providing an “intelligent underlay/overlay” (IUO) system. Using these techniques, a set of frequency channels are used with additional lower coverage channels, called “super-reuse channels”. The quality of each user is continuously monitored and when the quality is good enough (i.e., the user is close enough to a base station), the call is handed over to a super-reuse channel. These super-reuse channels can have lower reuse factors than normal channels and can be reused even in every cell. As mentioned above, though, spectrum availability is limited.
Thus, there is a need for improved continuous coverage in wideband high data rate systems. Further, there is a need to use variable bandwidth to increase re-use in a communication system. Even further, there is a need for higher frequency reuse without increasing the operator spectrum requirement.